The invention relates to a coating device for the continuous coating of driven web materials such as, for example, paper webs or metal webs, but preferably film webs of plastics or regenerated cellulose. The coating agents can be, for example, dispersions, emulsions, solutions, hot-melt adhesives and the like.
In the paper and film finishing industry, a plurality of very diverse technologies for coating travelling material webs has found acceptance. Thus, for example, a so-called doctor roll applicator, as described in German Offenlegungsschriften Nos. 1,577,647 and 2,034,004 is employed. FIG. 2 of the first-mentioned Offenlegungsschrift illustrates very well the mode of action of such processes. Thus, an excess of coating agent is first applied, by means of a feed roll which runs in the direction of, or counter to, the web travel and dips into an applicator trough filled with coating substance for applying the substance onto the substrate to be coated, for example a web of paper, film or metal. The necessary adjustment to give the particular desired coating weight is then effected immediately afterwards by a downstream doctor roll, usually a rotating circular rod which may be smooth or may sometimes also be wound with wires of very diverse thicknesses. This rod, in the latter case, also is known in the art under the English specialist term "wirebar".
These devices, for example exemplified by German Offenlegungsschrift No. 1,577,647, which employ freely mounted rods, the diameter of which is approximately of the order of 40 mm, however, suffer from disadvantages, predominately due to processing technology problems. Thus, for example, a contamination of the doctor rod by adhering, dried-out coating material is encountered, resulting in impairment of the quality of the coating, and the formation of streaks and imperfections.
As the art has progressed, exemplary devices which are customary at the present time are illustrated in German Offenlegungsschriften Nos. 2,034,004 and 2,307,404. Characteristic features of these systems are the substantially reduced doctor rod diameters, the mounting of the doctor rod in a doctor bed which supports it over its entire length and in part surrounds it, as well as devices for keeping the rotating rod clean, the latter devices being in the form of wiping edges and flushing grooves.
In spite of these developments, these devices still exhibit substantial shortcomings which hinder their universal application. A disadvantage is to be found, particularly, in the combination of the new metering device with the conventional feed system in the form of dip rolls, transfer rolls and/or applicator rolls. This circumstance manifests itself as a troublesome factor particularly in high speed coating using coating substances which are sensitive to external influences, for example polyvinylidene dispersions which are sensitive to shear and tend to form coagulate and foam. At the desired web speeds of 300 to 400 m/minute, the feed roll, which the machinery manufacturers traditionally like to make of large size for example with diameters of 200 mm and above, must of course also run at high speed in order to ensure that excess material is applied. The consequences are that, in a very short time, mountains of foam are generated in the applicator trough and, as a consequence, a blistered coating of inferior quality is produced.
However, attempted counter-measures in the form of a reduction of the diameter of the feed rolls have exactly the opposite effect. Smaller rolls must, since they have a lesser contact surface, be operated at correspondingly higher speeds of revolution with a view to comparable application weights of coating material compared with a large roll.
Modifying the roll surface by grooving, providing a screen-type surface, or equivalent approaches, which normally lead to the expectation of a higher feed rate, do not produce the desired effect from the point of view of enabling the lowering the speed of rotation of the rolls.
To circumvent the problems described, an emergency solution has been adopted in most cases; namely, constant and repeated circulation of the coating substance by pumping. However, this step undoubtedly amounts to an additional harmful treatment of the coating agent, even if the pumping circulation of the material is, inter alia, effected relatively gently by means of suitable conveying units such as membrane pumps.
In addition to the disadvantages described, emanating from the process, the actual doctor roll system also suffers from disadvantages resulting from the technical aspects of the apparatus. These include, inter alia, the preferred use of doctor beds made of plastics, such as, for example, polyethylene, polypropylene, polyamide, polytetrafluoroethylene and the like, and also of rubber-like materials.
The doctor bed profiles which are subject to wear and hence can only be employed for a limited time are either extruded as a profile, or cast, or produced from solid material by mechanical machining processes such as milling, planing, drilling and the like, and are produced in separate manufacturing operations. The fact that these manufacturing methods are costly is understandable, and in view of the fact that the spent doctors have to be thrown away, the methods are not economical, particularly since this situation influences the continuous total operating costs for coating, and since downtimes have to be tolerated.
It is not infrequently the case that the temperature-conditioning, that is to say both cooling and heating, of the coating substance and of the equipment is a technological necessity. The poor heat conductivity properties of the plastic doctor beds however in such a situation also prevent optimum operating temperatures being set up, which must be regarded as a further disadvantage of the conventional applicator systems.
A further shortcoming of the known apparatuses is to be seen in the flushing circuits for keeping the doctor rods clean.
Since, according to experience, no complete sealing of these systems is achievable, it is entirely possible that the flushing medium will come into contact with the coating substance. In the case of, for example, polyvinylidene coating, contact with water, which is the preferentially used flushing agent, can lead to precipitation of the dispersion. Obviously, similar occurrences can also not be excluded in the case of reactive adhesive mixtures or solutions. The coagulate sludge thus produced on the one hand blocks the flushing channels and can moreover even be carried by the rotating doctor rod into the plastic bed, making the latter unusable.
Accordingly, the object presented itself of developing a coating device which constitutes a technical advance and in which the indicated disadvantages and shortcomings of the device of the prior art are avoided.